Operation of OSPFv3
| OSPFv3 is specified in RFC 2740. There are some high-level similarities between the relationships of RIPng to RIPv2 and OSPFv3 to OSPFv2. Most important, OSPFv3 uses the same fundamental mechanisms as OSPFv2the SPF algorithm, flooding, DR election, areas, and so on. Constants and variables such as timers and metrics are also the same. Another similarity to the relationship of RIPng to RIPv2 is that OSPFv3 is not backward-compatible with OSPFv2. So if you want to use OSPF to route both IPv4 and IPv6, you must run both OSPFv2 and OSPFv3. As this chapter is being written, there is discussion of adding IPv4 support to OSPFv3, but no specifications have yet been completed. I, for one, hope this support is added, because OSPFv3 is a significantly improved protocol. It is assumed that you have read the previous chapter and understand the operation of OSPFv2. This section does not repeat that information, but presents the significant differencesprimarily operational and LSA formatsof OSPFv3. OSPFv3 Differences from OSPFv2In addition to the changes in LSAs, described in the next section, there are several changes to OSPF procedures themselves. This section describes the most important of those changes:
OSPFv3 MessagesOSPFv2 and OSPFv3 both have the same protocol number of 89, although OSPFv3, being an IPv6 protocol, more accurately has a Next Header value of 89. And like OSPFv2, OSPFv3 uses multicast whenever possible. The IPv6 AllSPFRouters multicast address is FF02::5, and the AllDRouters multicast address is FF02::6. Both have link-local scope. You can easily see the similarity in the last bits with the OSPFv2 addresses of 224.0.0.5 and 224.0.0.6. OSPFv3 uses the same five message typesHello, DD, LS Database Request, LS Database Update, and LS Acknowledgmentas OSPFv2, and numbers them the same. The message header, shown in Figure 9-1, is somewhat different from the OSPFv2 message header. The version number, of course, is 3 rather than 2. But more important, there are no fields for authentication. As discussed in the previous section, OSPFv3 uses the Authentication extension header of the IPv6 packet itself rather than its own authentication process. And there is an Instance ID, which allows multiple OSPFv3 instances to run on the same link. The Instance ID has local link significance only, because the OSPFv3 message is not forwarded beyond the link on which it is originated. Figure 9-1. OSPFv3 packet header. Aside from the message header, the format of only two of the OSPFv3 messages is different from their OSPFv2 counterparts: Hellos and Database Description messages. Figure 9-2 shows the format of the OSPFv3 Hello message. Unlike OSPFv2, there is no Network Mask field because IPv6 has no need for it. Beyond that, the same fields (below the header) appear in both packets. The Options field increases in size to 24 bits, and the Router Dead Interval is decreased from 32 bits to 16 bits. The implication of this second field is that the theoretical maximum Router Dead interval is decreased from 4.3 billion seconds to 65,535 seconds. This change has little or no bearing on operational networks. The maximum configurable Router Dead interval on the most common OSPF implementations has long been 65,535 anyway, and intelligent OSPF designs do not approach even this maximum; so the change in the field size is only for conservation of unused space. Figure 9-2. OSPFv3 Hello message. Figure 9-3 shows the format of the OSPFv3 Database Description packet. It differs from its OSPFv2 counterpart only in the larger Options field. Figure 9-3. OSPFv3 Database Description message. The other three message typesLink State Request, Link State Update, and Link State Acknowledgmenthave formats that do not differ from those of OSPFv2, and so are not shown in this chapter. An Overview of OSPFv3 LSAsThe OSPFv3 LSA header is shown in Figure 9-4. Comparing it with the OSPFv2 LSA header in Figure 8-54, you can see that it is almost identical, except that there is no Options field, and the Link State Type field is 16 bits rather than the 8-bit Type field of OSPFv2. Figure 9-4. OSPFv3 LSA header. The reason that the Link State Type field is longer is that it includes three preceding bits, as shown in Figure 9-5. Figure 9-5. Link State Type field of the OSPFv3 LSA header. U indicates how a router should handle the LSA if it does not recognize the LSA's Function Code. If the bit is cleared, the unknown LSA is to be treated as if it had link-local flooding scope. If the U bit is set, the unknown LSA is to be stored and flooded as if it is understood. S2 and S1 indicate the LSA's flooding scope. Table 9-1 shows the possible values of these two bits and the associated flooding scopes.
LSA Function Code, the last 13 bits of the LS Type field, corresponds to the OSPFv2 Type field. Table 9-2 shows the common LSA types used by OSPFv3 and the values of their corresponding LS Types. If you decode the hex values, you will see that the default U bit of all of them is 0. The S bits of all LSAs except two indicate area scope. Of the remaining two, AS External LSAs have an AS flooding scope and Link LSAs have a link-local flooding scope. Most of the OSPFv3 LSAs have functional counterparts in OSPFv2; these OSPFv2 LSAs and their types are also shown in Table 9-2.
Although Router and Network LSAs have the same names, there is a significant difference in how the OSPFv3 and OSPFv2 Router and Network LSAs function. Specifically, OSPFv3 Router and Network LSAs do not advertise prefixes. This is an important improvement in the scaling of the protocol. These LSAs are, as you know from Chapter 8, "OSPFv2," primarily to represent the router as a node on the SPF tree. So when a Router or Network LSA is flooded, there is an assumption that a topological change has taken place and all routers in the area, on receipt of the LSA, rerun SPF. But because OSPFv2 routers also use these LSAs to advertise their connected subnets, if a subnet changes, the LSA must also be flooded to advertise the change. Even though something like an address change does not affect the SPF topology, the reception of a Router or Network LSA triggers an SPF run anyway. This can be particularly problematic for edge or access routers that have many stub links that change regularly. OSPFv3 removes the prefix advertisement function from Router and Network LSAs, and puts it in the new Intra-Area Prefix LSA. Now Router and Network LSAs only represent the router's node information for SPF and are only flooded if information pertinent to the SPF algorithm changes. If a prefix changes, or the state of a stub link changes, that information is flooded in an Intra-Area Prefix LSA that does not trigger an SPF run. Another difference between Router and Network LSAs between OSPFv2 and OSPFv3 is in the exchange of some information that is only relevant to directly connected neighbors. OSPFv2 puts this information in Router or Network LSAs; and although only the directly connected neighbors care about the information, it is flooded with the LSAs throughout the area. OSPFv3 takes this neighbor-specific information and puts it in the new Link LSA, which has link-local flooding scope. Although minor, the introduction of the Link LSA does add an efficiency improvement over OSPFv2. Inter-Area Prefix, Inter-Area Router, and Type-7 LSAs, although their names have changed, have the same function as OSPFv2 Network Summary, ASBR Summary, and NSSA LSAs, respectively. AS-External and Group Membership LSAs have both the same names and the same functions in both OSPFv2 and OSPFv3. OSPFv3 LSA FormatsThis section details the formats of the first five OSPFv3 LSA types, and the two new LSA types shown in Table 9-2. Although there is a specified Group Membership LSA for Multicast OSPF (MOSPF), that protocol is not covered in this book, and so the LSA is also not detailed. In most cases, only fields that differ from their OSPFv2 counterparts are discussed; you are encouraged to compare those LSAs that have corresponding OSPFv2 LSAs with the figures in Chapter 8. The Router LSAFigure 9-6 shows the format of the OSPFv3 Router LSA. As discussed in the previous section, prefix information is not included in the Router LSA as it is in its OSPFv2 counterpart. The OSPFv3 Router LSA only describes the originating router and its links to neighbors, for use in SPF calculations. Prefix information is carried in the Intra-Area Prefix LSA. Figure 9-6. OSPFv3 Router LSA. Also notice that the ToS metric fields, obsolete already in OSPFv2, are not included in this LSA. Options, although in a different position within the LSA format and a longer length (24 bits) than the OSPFv2 Options field, performs the same function of identifying optional capabilities. The Options field, carried in several packets and LSAs, is described separately in a later section. Following the Options field is a set of fields that can appear multiple times, to describe each of the attached interfaces:
Network LSAThe OSPFv3 Network LSA is shown in Figure 9-7. Functionally it is identical to the OSPFv2 Network LSA (originated by the DR to represent a pseudonode, 0 cost to attached neighbors is assumed, and so on). The only significant differences are the location of the Options field and the elimination of the Network Mask field, which has no meaning in IPv6. Figure 9-7. OSPFv3 Network LSA. Inter-Area Prefix LSAThe Inter-Area Prefix LSA (Figure 9-8) performs the same function as the OSPFv2 type 3 Summary LSAABRs originate them into an area to advertise networks that are outside the area but inside the OSPF domain. Only the name, more descriptive in OSPFv3, has changed. An ABR originates a separate Inter-Area Prefix LSA for each IPv6 prefix that must be advertised into an area. An ABR can also originate an Inter-Area Prefix LSA to advertise a default route into a stub area. Figure 9-8. OSPFv3 Inter-Area Prefix LSA. Inter-Area Router LSAThe Inter-Area Router LSA performs the same duties for OSPFv3 as the type 4 Summary LSA performs for OSPFv2. An ABR originates an Inter-Area Router LSA into an area to advertise an ASBR that resides outside of the area. The ABR originates a separate Inter-Area Router LSA for each ASBR it advertises. Figure 9-9 shows the format of the Inter-Area Router LSA. Even though the OSPFv2 type 3 and type 4 Summary LSAs have the same formats, you can see by comparing Figure 9-8 and Figure 9-9 that the Inter-Area Network and Inter-Area Router LSAs are different. The fields are self-descriptive:
Figure 9-9. OSPFv3 Inter-Area Router LSA. AS-External LSAAs with OSPFv2, the AS-External LSA advertises prefixes external to the OSPF routing domain; one LSA is required for each external prefix advertised. However, the format of the OSPFv3 As-External LSA (Figure 9-10) is different from its OSPFv2 counterpart. Figure 9-10. OSPFv3 AS-External LSA. The E flag performs the same function in the OSPFv3 LSA as in the OSPFv2 LSA. If set, the metric is a type 2 external metric. If the bit is cleared, the metric is a type 1 external metric. The F flag indicates, when set, that a forwarding address is included in the LSA. The T flag indicates, when set, that an external route tag is included in the LSA. Metric, of course, specifies the cost of the route. Whether it is type 1 or type 2 depends on the value of the E flag. Prefix Length, Prefix Options, and Address Prefix completely describe the enclosed prefix. Forwarding Address, if included, is a fully specified 128-bit IPv6 address representing the next-hop address to the destination prefix. It is included only if the F flag is set. External Route Tag, if included, performs the same function as the External Route Tag field in the OSPFv2 AS-External LSA. It is included only if the T flag is set. Referenced Link State ID and Referenced Link State Type, if included, allow additional information about the prefix to be included in another LSA. If used, these two fields describe the link state ID and the type of the LSA carrying the additional information. The Advertising Router field of the referenced LSA must also match the value of the Advertising Router field in the AS-External LSA. The additional information has, as with the External Route Tag, no relevance to OSPFv3 itself, but is used to communicate information across the OSPF domain between border routers. If this function is not used, the Referenced Link State Type field is set to all zeroes. Link LSAThe Link LSA is used for communicating information that is significant only to two directly connected neighbors. The format of the Link LSA is shown in Figure 9-11. A separate Link LSA is originated on each of a router's attached links belonging to an OSPFv3 domain, and a receiving router, because of the link-local flooding scope, never forwards the LSA to any other link. Figure 9-11. OSPFv3 Link LSA. The LSA performs three functions:
Router Priority specifies the router priority assigned to the interface of the originating router. Options specifies the options bits that the originating router would like to set in the Network LSA that will be originated for the link. This is the same 24-bit Options field carried in OSPFv3 Hello and DD packets, in a number of OSPFv3 LSAs. The Options field is detailed in the later section, "The Options Field." Link-Local Prefix Address specifies the 128-bit link-local prefix of the originating router's interface attached to the link. Number of Prefixes specifies the number of IPv6 prefixes contained in the LSA, as described by the following Prefix Length, Prefix Options, and Address Prefix fields. Prefix Length, Prefix Options, and Address Prefix describe one or more IPv6 prefixes associated with the link by the originating router. This set of fields is used not only in the Link LSA, but also the Intra-Area-Prefix, Inter-Area-Prefix, and AS-External LSAs. The advertised prefix can be any length between 0 and 128. When the prefix is not an even multiple of 32 bits, it is padded out with zeroes to fit the 32-bit boundaries of the Address Prefix field. The Prefix Length field specifies the length of the unpadded prefix, in bits. The Prefix Options field, shown in Figure 9-12, specifies optional handling of the prefix during routing calculations. Figure 9-12. Prefix Options field. The Propagate (P) bit is set on NSSA area prefixes that should be re-advertised at the NSSA area border. The Multicast (MC) bit, when set, specifies that the prefix should be included in multicast routing calculations. The Local Address (LA) bit, when set, specifies that the prefix is an interface address of the advertising router. The No Unicast (NU) bit, when set, specifies that the prefix should be excluded from unicast route calculations. Intra-Area Prefix LSAThe Intra-Area Prefix LSA is shown in Figure 9-13. Recall from the previous discussion of this LSA that attached prefixes, which in OSPFv2 are carried in Router and Network LSAs, in OSPFv3 are carried in Intra-Area Prefix LSAs. Prefix changesincluding additions and deletionsdo not influence the branches of the SPF tree in any way. But OSPFv2 advertises prefixes in Router and Network LSAs, causing an SPF calculation in all routers in the area whenever a prefix change occurs. With OSPFv3, however, when a link or its prefix changes, the connected router originates an Intra-Area Prefix LSA to flood the information throughout the area. This LSA does not trigger an SPF calculation; the receiving routers simply associate the new prefix information with the originating router. Router and Network LSAs, in OSPFv3, serve only to provide topological information. As a result, this new LSA should make OSPFv3 significantly more scalable for networks with large numbers of frequently changing prefixes. Figure 9-13. Intra-Area Prefix LSA.
If the prefixes are associated with a Router LSA, the Referenced Link State Type is 1, the Referenced Link State ID is 0, and the Referenced Advertising Router is the RID of the originating router. If the prefixes should be associated with a network LSA, the Referenced Link State Type is 2, the Referenced Link State ID is the interface ID[1] of the link's DR, and the Referenced Advertising Router is the RID of the DR.
Each prefix is then represented by a Prefix Length, Prefix Options, and Address Prefix field, as described previously for the Link LSA. Added to these three fields is the Metric field, which is the cost of the prefix. Options FieldThe 24-bit Options field, specifying optional capabilities of the originating router, is carried in Router, Network, Inter-Area Router, and Link LSAs. This field is also carried in the Hello and Database Description packets. Figure 9-14 shows the format of the Options field. As of this writing, only the six rightmost bits are defined as options flags, and most of those are the same flags you are familiar with from OSPFv2. OSPFv3 routers ignore unrecognized options flags. Figure 9-14. Options field.
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